The gravitational force of attraction between two massive bodies is the force which results from the inherent natural attraction between the two bodies. The magnitude of the gravitational force is related to the mass of the bodies and is inversely related to the separation distance between centers of mass of the bodies. Gravity is measured as acceleration. For instance, and in general, 9.8 m/s2 is the acceleration that a 1 gram massive object will fall toward the ground when it is thrown from the roof of a building.
A gravimeter is an instrument used to measure the strength or magnitude of gravity. Gravimeters are well known and typically measure the vertical component of the total gravity vector of the earth in units of acceleration. The common unit of measurement of gravity is the “gal,” which is a unit of acceleration defined as 1 gal=1 cm/s2=0.01 m/s2=10−3 g. The typical gravimeter currently measures gravity to the nearest microgal (1 μgal=10−9 g). These types of measurements are referred to herein simply as “gravity measurements.”
Gravity measurements are useful for a number of different purposes, as illustrated by the following examples. Gravity measurements are used to monitor subsurface density changes resulting from immediate to long-term subterranean events. Gravity measurements are also used to monitor the influx of water when flooding a petroleum reservoir to push hydrocarbon into extraction wells. The confinement of waste gas and liquid substances stored in subsurface caverns or containments are monitored by gravity measurements to detect whether the waste and liquid substances remain securely confined. Water management techniques make use of gravity measurements to monitor the extent to which groundwater moves or the extent to which rainwater penetrates into and saturates the soil.
In all of these uses, changes in the quantity of the substance, such as the oil, water or gas, alter the density of the volume of material at the monitored location. That change in mass, through Newton's law, changes the gravity around and above that specific monitored location. For example, in the absence of any other change, the depletion of petroleum from a subterranean reservoir decreases gravity at the point above the reservoir due to the reduction of petroleum in the reservoir. Similarly, the introduction of water to flood a petroleum reservoir increases gravity above those parts of the reservoir where the water has penetrated and replaced a less dense substance or filled a void. The movement of groundwater and waste substances from their previous location is sensed by measuring temporal changes in gravity at specific points surrounding the location. By measuring gravity, the extent of movement of the substance can be determined, and with appropriate accuracy of the measurements, volumetric quantities of the substance can also be determined.
The change in mass of the monitored substance is normally very small compared to the mass of the surrounding earth that defines the reservoir or the cavern or confinement of the substance, so the change in gravity is usually very small. Nevertheless, the change does occur and gravimeters are capable of measuring such relatively small changes in gravity. Gravimeters fall into two categories: a relative gravity measurement instrument known as a relative gravimeter, and an absolute gravity measurement instrument known as an absolute gravimeter. Both types of gravimeters measure the vertical component of the earth's total gravity vector.
A relative gravimeter suspends a mass of known quantity by a spring-like device. An increase in gravity interacts with the known mass in such a way to slightly stretch or elongate the spring-like device. Conversely, a decrease in gravity allows the spring-like device to constrict slightly. In either case, the position of the known mass changes by a slight amount due to the elongation or constriction of the spring-like device. The amount of physical displacement of the known mass is directly related to the magnitude of gravity at that location and time.
An absolute gravimeter is a technically sophisticated, more expensive and physically larger instrument than a relative gravimeter, at the present time. A mass of known quantity is positioned within a chamber which has been evacuated as much as possible to approximate a complete vacuum. A mechanism lifts the known mass and releases it to freefall within the chamber. A laser beam monitors movement of the free falling mass, and an extremely accurate clock measures the time required for the mass to fall a specific distance or measures the speed of the free falling mass at a specific time. By utilizing the distance and/or speed data, the magnitude of gravity acting upon the known mass at the time of the test is calculated.
In a relative gravimeter, the spring-like device which suspends the known mass is susceptible to many influences that degrade the accuracy of the gravity measurements obtained. Changes in temperature of the spring-like device can change its spring characteristics and hence change the displacement of the known mass. Changes in pressure can also change its spring characteristics. Shocks caused by physical movement of the gravimeter can cause an offset or tare in the at-rest position of the known mass. Changes in the spring characteristics of the spring-like device naturally occur over time and are referred to as drift. If these external environmental and characteristic influences and variations are not recognized and corrected, these influences and variations might incorrectly be interpreted as a change in gravity when measuring temporal changes in gravity.
Relative gravimeters are generally less reliable and less accurate than absolute gravimeters for measuring temporal changes in gravity. This is particularly the case when relatively small gravity influences are measured, such as subsurface density changes in subterranean reservoirs. The effect of changes in temperature, pressure, tare or drift can mask any change in the magnitude of gravity, making it impossible to accurately measure small changes in gravity.
While the accuracy of the gravity measurements obtained by using an absolute gravimeter is very high compared to the accuracy in measurements obtained by using a relative gravimeter, the sensitivity and complexity of the absolute gravimeter has made it impossible or very difficult to employ a gravimeter in any location other than in a controlled scientific laboratory where all of the environmental influences can be controlled. Only recently have field-usable absolute gravimeters been developed, but such field-usable absolute gravimeters are expensive, in the neighborhood of US $300,000-500,000, which is roughly five times the price for a relative gravimeter. Furthermore, the use of such field-usable absolute gravimeters is tedious and time-consuming.
The relatively high cost of field-usable absolute gravimeters has had the practical consequence of requiring a single absolute gravimeter to be moved from one location to another, in order to acquire absolute gravity measurements at each of the locations. Even though movable, the sensitivity and fragility of a field-usable absolute gravimeter complicates its use. The set-up time at each location is lengthy, and care must be taken in moving the absolute gravimeter from one location to avoid damage. These factors have the effect of limiting the number of points where gravity can be measured within a specific amount of time. Under typical circumstances, absolute gravity measurements can be made at only two or three different locations in a day. As many as 300 different absolute gravity measurements may be required to complete a gravity survey of a typical petroleum reservoir. Consequently, a relatively long time is required to complete a typical absolute gravity survey.
Each absolute gravity measurement is also time-specific, becoming in effect, a “snapshot” of the gravity that exists at the time when the measurement is made. By the time that an absolute gravity survey of a petroleum reservoir is completed, the measurements can only represent an average of the gravity magnitude over the many days required to complete the survey. Because of the logistical planning and cost involved in conducting an absolute gravity survey, absolute gravity surveys are conducted quite infrequently, typically separated by one year intervals between surveys. The long time between the absolute gravity surveys results in low resolution in the ability to monitor subsurface events over time, even though the gravity measurements are very accurate when made.
Relative gravimeters are more commonly used to conduct gravity surveys. A relative gravimeter is moved from one measurement point to the next measurement point and the change in the gravity between measurement points is recorded. Periodically and before completing all the measurement points, the relative gravimeter is moved back to a reference point, referred to as a base station. The difference in the sequential measurements at the base station is distributed among the measurements collected between the sequential occupations of the base station. This type of gravity survey, where the relative gravimeter is periodically returned to the reference point to derive an error or correction value, is known as “looping.” In looping relative gravity surveys, only relative or comparative gravity is measured from one point to the next, including at the base station reference point.
The comparative gravity measurements obtained in a looping relative gravity survey are used as a surrogate for absolute gravity, under the assumption that absolute gravity must be related to the comparative gravity. While there is some validity and utility to this assumption, the accuracy of the gravity measurements made in a looping gravity survey is nevertheless subject to variable influences of temperature, pressure, tare and drift on the relative gravimeter. Moreover, the time and logistics required to accomplish the looping gravity survey can be significant, particularly in rough terrain or in active locations, such as producing oil or gas fields. The added time to perform the looping relative gravity survey also diminishes the ability to monitor dynamic changes in comparative gravity.
There are many desirable reasons for determining subsurface density information on a fairly rapid, dynamic or real-time basis. For example, rapid changes in the gravity field may be indicative of water breakthroughs or leakage of waste substances from their intended locations. Both absolute and relative gravimeters have the capability of responding to more dynamic events, but their acquisition costs, drift characteristics and other factors have limited their practical use for such purposes. Consequently, accurate and dynamic gravity information for gravity surveys has not previously been available on an economic basis.